From d730fdd17c644a2a048af32c47d2cbfc6a997ee7 Mon Sep 17 00:00:00 2001
From: leaysgur <6259812+leaysgur@users.noreply.github.com>
Date: Tue, 6 Oct 2026 02:57:14 +0000
Subject: [PATCH] fix(formatter_core): leave blank frontmatter without
 dispatcher (#27359)

Always leave FM (in any form) as-is when `embeddedLanguageFormatting: off` (or Rust CLI).
---
 apps/oxfmt/test/api/front_matter.test.ts             |  7 +++++++
 crates/oxc_formatter_core/src/envelope/mod.rs        | 12 ++++++------
 crates/oxc_formatter_css/src/format.rs               |  2 +-
 crates/oxc_formatter_css/tests/conformance.rs        |  3 ++-
 .../tests/snapshots/conformance__prettier-css.snap   |  2 +-
 5 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/apps/oxfmt/test/api/front_matter.test.ts b/apps/oxfmt/test/api/front_matter.test.ts
index 2aa4e83089..4be0de8364 100644
--- a/apps/oxfmt/test/api/front_matter.test.ts
+++ b/apps/oxfmt/test/api/front_matter.test.ts
@@ -68,6 +68,13 @@ describe("CSS front matter", () => {
     });
     expect(result.errors).toStrictEqual([]);
     expect(result.code).toBe("---\ntitle:   Home\n---\n\nb {\n}\n");
+
+    // Even an empty block is not normalized
+    const empty = await format("a.css", "---\n\n---\nb {}\n", {
+      embeddedLanguageFormatting: "off",
+    });
+    expect(empty.errors).toStrictEqual([]);
+    expect(empty.code).toBe("---\n\n---\n\nb {\n}\n");
   });
 
   it("keeps a physical BOM at byte 0, before the block", async () => {
diff --git a/crates/oxc_formatter_core/src/envelope/mod.rs b/crates/oxc_formatter_core/src/envelope/mod.rs
index cdd5197f6c..3520fb0028 100644
--- a/crates/oxc_formatter_core/src/envelope/mod.rs
+++ b/crates/oxc_formatter_core/src/envelope/mod.rs
@@ -18,13 +18,13 @@ use crate::{
 
 /// Writes a front matter block.
 ///
-/// The body dispatches through the session when its resolved language is in `embeddable_languages`,
-/// the whole block stays verbatim otherwise.
+/// The body dispatches through the session when a dispatcher is installed
+/// and its resolved language is in `embeddable_languages`, the whole block stays verbatim otherwise.
 /// Membership means "gets the frame treatment", not "gets formatted":
 /// a member language without a serving formatter keeps its body verbatim (`PreserveOriginal`),
-/// but its EMPTY block still normalizes through the frame.
-/// Never fails; any refusal (`PreserveOriginal`, operational error, non-embeddable language) keeps the block's bytes as-is
-/// while the host document still formats.
+/// but its EMPTY block still normalizes through the frame when a dispatcher is installed.
+/// Never fails; any refusal (`PreserveOriginal`, operational error, no dispatcher, non-embeddable language)
+/// keeps the block's bytes as-is while the host document still formats.
 ///
 /// The composed shape (Prettier `embed.js` + its css/markdown printers):
 /// opening delimiter with the explicit language re-emitted (`---yaml`),
@@ -36,7 +36,7 @@ pub fn write_front_matter<'a, C: FormatContext>(
     f: &mut Formatter<'_, 'a, C>,
 ) {
     let language = fm.language();
-    if embeddable_languages.contains(&language) {
+    if f.session().has_dispatcher() && embeddable_languages.contains(&language) {
         let body = fm.value.trim();
         if body.is_empty() {
             write_frame(fm, None, f);
diff --git a/crates/oxc_formatter_css/src/format.rs b/crates/oxc_formatter_css/src/format.rs
index e02098a038..8389efcd38 100644
--- a/crates/oxc_formatter_css/src/format.rs
+++ b/crates/oxc_formatter_css/src/format.rs
@@ -31,7 +31,7 @@ pub fn format<'a>(
     options: CssFormatOptions,
 ) -> Result<Formatted<'a, CssFormatContext<'a>>, OxcDiagnostic> {
     // NOTE: this wrapper labels the run `PhysicalFile` with NO services:
-    // front matter is detected but its body degrades to verbatim (`PreserveOriginal`),
+    // front matter is detected but kept verbatim (no dispatcher),
     // and `@apply` Tailwind classes print unsorted.
     // Hosts that want them use `format_with_session` with the services installed.
     format_with_session(
diff --git a/crates/oxc_formatter_css/tests/conformance.rs b/crates/oxc_formatter_css/tests/conformance.rs
index 7d6a742882..aae856aa49 100644
--- a/crates/oxc_formatter_css/tests/conformance.rs
+++ b/crates/oxc_formatter_css/tests/conformance.rs
@@ -26,8 +26,9 @@ const CSS: ConformanceConfig = ConformanceConfig {
     ignore: &[
         // postcss-conditionals (archived: https://github.com/andyjansson/postcss-conditionals).
         "css/atrule/if-else.css",
-        // YAML frontmatter
+        // YAML frontmatter, needs the dispatcher this harness doesn't install
         "css/yaml/dirty.css",
+        "css/yaml/empty_newlines.css",
         // range formatting / IDE cursor, not whole-file formatting
         "css/range/",
         "css/cursor/",
diff --git a/crates/oxc_formatter_css/tests/snapshots/conformance__prettier-css.snap b/crates/oxc_formatter_css/tests/snapshots/conformance__prettier-css.snap
index 09c58f8cc8..0dc69637df 100644
--- a/crates/oxc_formatter_css/tests/snapshots/conformance__prettier-css.snap
+++ b/crates/oxc_formatter_css/tests/snapshots/conformance__prettier-css.snap
@@ -2,7 +2,7 @@
 source: crates/oxc_formatter_css/tests/conformance.rs
 expression: report
 ---
-css compatibility: 142/151 (94.04%), 13 files skipped
+css compatibility: 141/150 (94.00%), 13 files skipped
 
 # Failed